The present disclosure relates to a method of producing an optical fiber preform and an optical fiber.
By realizing a digital coherent transmission technology, speed and capacity in optical fiber transmission have been improved. Along with this, higher signal-to-noise ratio (SNR) is demanded in an optical transmission system, and a lower transmission loss property has been needed for an optical fiber forming a transmission path in the optical transmission system. In order to reduce a transmission loss of the optical fiber, it is effective to reduce a Rayleigh scattering produced in the optical fiber. The Rayleigh scattering is caused by a fluctuation in dielectric permittivity of a glass and involves two factors of composition fluctuation and density fluctuation. The density fluctuation depends on a solidification temperature (fictive temperature) at which it is solidified as a glass from a supercooled liquid state. It is known that a Rayleigh-scattering loss may be reduced by doping a core portion of an optical fiber with an alkali metal since the fictive temperature decreases evidently (See U.S. Pat. No. 5,146,534).
The below-described methods have been proposed as a method of doping the core portion of the optical fiber made of a silica glass with the alkali metal.
For example, a method has been proposed for manufacturing an alkali-metal-doped preform by synthesizing an alkali-metal-doped silica core soot by a vapor phase axial deposition (VAD) method or outside vapor deposition (OVD) method and by dehydrating and vitrifying the core soot (see U.S. Pre-Grant Publication No. 2004/0206127). Moreover, there have been proposed a method of introducing an alkali metal vapor into a silica furnace tube containing a silica core soot to infiltrate the alkali metal to dope the silica core soot therewith (see Japanese Laid-open Patent Publication No. 2013-199399) and a method of diffusing and doping alkali metal from an outer surface of a silica core preform introduced into a silica furnace tube (see U.S. Pre-Grant Publication No. 2005/0129376). Further, there has been proposed a method of diffusing and doping an alkali metal from an inside of a silica tube to be a silica core preform by modified chemical vapor deposition (MCVD) method (see Japanese Laid-open Patent Publication No. 2012-162409).
However, in the method of synthesizing the silica core soot doped with the alkali metal as disclosed in U.S. Pre-Grant Publication No. 2004/0206127, the alkali metal introduced into a silica soot is converted into a chloride and volatilized easily by a chlorine-based gas flown at a subsequent dehydration step. Therefore, it is difficult to leave the alkali metal at the core portion at a desirable concentration. In the method of making the alkali metal infiltrated to the silica core soot as disclosed in Japanese Laid-open Patent Publication No. 2013-199399, the silica core soot is doped with impurities such as a transition metal or the like contained in alkali metal material together with the alkali metal. In order to remove the impurities, it is effective to infiltrate the alkali metal to the silica core soot and then perform a dehydrating-and-purifying step using chlorine. However, in this case, since the alkali metal tends to be volatilized for the above-described reason, it is difficult to obtain a desirable doping amount of alkali-metal. On the other hand, in the method as disclosed in US Pre-Grant Publication No. 2005/0129376 which diffuses the alkali metal from the silica core preform's surface, there is a problem of decreased doping efficiency since most of the used alkali metal is exhausted to outside a furnace without being diffused in the silica core member. In the method as disclosed in Japanese Laid-open Patent Publication No. 2012-162409 which diffuses and dopes the alkali metal from an inside of a silica tube, there is a tendency of ovalization of the silica tube by a decreased viscosity accompanying an increase in the silica tube's temperature in a step of diffusing the alkali metal to an inside of the silica tube from an alkali metal layer deposited on a silica tube's inner wall. Therefore, it is difficult to maintain the silica tube at a higher temperature for a long time, and thus, an amount of diffusion of the alkali metal into the inside of the silica tube is limited. In addition, since an alkali metal layer with a higher concentration is deposited on inner wall of the silica tube, a crystal phase (cristobalite) is generated with the alkali metal as a core, and thus, there is a problem that a fine crack is produced easily on the surface of the glass.
There is a need for a method of producing an optical fiber preform which realizes easily and effectively doping a core portion with a desirable concentration of alkali metal and producing an optical fiber preform having very low impurity concentration, and a method of producing an optical fiber which may easily produce an optical fiber having low transmission loss.